Heterogeneous catalysis processes, using metal catalysts, are of commercial importance in a large number of chemical and petrochemical processes. In all cases, the economic performance of the processes depends, to a greater or lesser extent, on the activity of the catalyst, its selectivity towards the desired reaction product, and the cost and complexity of preparation of the catalyst in its most advantageous form for use in the particular process under consideration. For most efficient catalysis, the catalyst should have large metallic surface area, i.e. a large surface to bulk ratio. This is achieved by producing the catalyst comprising individual occurrences of metal atoms (monatomic) where most of the metal atoms are atoms in the zero-valent form and wherein the individual occurrences of metal atoms (monatomic) form aggregates or clusters of up to 100 atoms per cluster. As used herein the term "cluster" refers to metal atoms weakly or strongly coupled, through space or through a support, a significant proportion of the metal atoms being in the zero-valent state and generally separated by a distance of six Angstroms (.ANG.) or less. Such a cluster includes any aggregation of two or more metal atoms, of the same or different species, regardless of whether they occur in substantially one dimensional form (i.e. a chain of metal atoms), or two-dimensional form (i.e. a planar arrangement), a spiral arrangement or a three dimensional structure.
When bulk metals, especially transition metals, are vaporized e.g. by resistive heating, the initially formed vapor is in the monoatomic condition. Very rapidly indeed, under normal conditions, the single metal atoms agglomerate into small clusters on a surface, and then very rapidly bulk, colloidal metal is formed by further agglomeration.
U.S. Pat. No. 4,292,253, Ozin et al, issued Sept. 29, 1982, describes a process for preparation of a catalyst in which the catalytic metal is present, in significant amounts, in small cluster form and is stable at or near room temperatures. The process described involves the generation of vapors of the metal in a high vacuum environment and in the vicinity of a liquid polymer, so that the metals are effectively "trapped" by the polymer in monatomic or small cluster form and prevented from recombining to form colloidal metal.
Zeolites are a well-known and widely used type of heterogeneous catalyst, especially in connection with hydrocarbon reactions. Zeolites, also known as molecular sieves, are detailed in the texts "Zeolite Molecular Sieves" by D. W. Breck, John Wiley and Sons, Inc., N.Y., N.Y. (1974), "Chemistry of Catalytic Processes" by B. C. Gates, J. R. Katzer, and G. C. A. Schuit, McGraw-Hill, N.Y., N.Y, (1979), "Fluid Catalytic Cracking with Zeolite Catalysts" by P. B. Venuto and E. T. Habib, Jr., Marcel Dekker Inc., N.Y., N.Y., (1979) and "Heterogeneous Catalysts in Practice" by C. N. Satterfield, McGraw-Hill, N.Y., N.Y. (1980). Advances in zeolite technology and applications based on U.S. Patents issued since 1977 are described by J. Scott in "Zeolite Technology and Applications, Recent Advances", Noyes Data Corp., Park Ride, N.J. (1980). A proceeding of an international symposium is "Catalysis by Zeolites" by B. Imelik et al, Elsevier Scientific Publishing Co., N.Y., N.Y. (1980). P. Gallezot describes "the State and Catalytic Properties of Platinum and Palladium in Faujasite-Type Zeolites" in Catal. Rev.-Sci. Eng., 20, 121-154 (1979).
Zeolites are basically alumino silicates of alkali or alkaline earth metals, initially combined with substantial amounts of water of hydration. They possess a characteristic crystal structure which derives from the tetrahedral configuration of the SiO.sub.4 unit which they contain, and the sharing of oxygen atoms from these tetrahedra with AlO.sub.4 tetrahedra also present. Alkali or alkaline earth metal ions associate with the crystal lattice structure to satisfy the resultant electrostatic charge. The result is a three dimensional crystal structure having therein major and minor cavities, connected by channels, both the cavities and the channels being of substantially constant narrowly defined size in any given zeolite material. The channels and the cavities are of a size comparable to the dimensions of molecules. Small molecules may enter the cavities, and hence penetrate the pores of the zeolite whereas larger molecules may not--thus the materials can act as sieves, to separate molecules and hence to separate molecular mixtures into their component parts. Any reactants deposited on the inside surface of the cavity will only contact and react with molecules small enough to enter the cavity, thus offering the possibility of arranging for highly selective chemical reactions. Zeolite molecular sieves can be subjected to ion exchange reactions whereby the alkali or alkaline earth metal present in ionic form is exchanged for a different ionic species. Since normally much of the pore volume of the crystalline zeolite is occupied by water of hydration, the zeolite must be dehydrated, without destroying the crystal lattice structure, before it can normally be used as an effective catalyst.
D. Frenkel and B. G. Gates (J. Am. Chem. Soc., 102, 2478 (1980)) describe zeolite-encapsulated cobalt clusters prepared by the reduction of Co (II) cations exchanged into the A and Y type zeolites. Furthermore, metal vapors, especially cadmium vapors, were used to reduce the cobalt (II) cation to zero-valent cobalt thus leaving ionic cadmium species and metallic cobalt wwithin the zeolite cavity.
In the present invention, metal in the zero-valent (atomic) state is deposited in and on the zeolite. No reducing agent is needed, hence no other metal cation such as cadmium, as a result of reduction-oxidation, is co-deposited in and on the zeolite. No significant amounts of metallic contaminants, deriving from co-deposition agents, remain in the catalyst. Moreover, the manner in which the catalytically active metal is deposited by low pressure metal vapor deposition is entirely different.